Although glass is a brittle material, the intrinsic strength of pristine glass optical fibers is very high, on the order of 1,000,000 psi for SiO.sub.2 based fibers. Typically, glass optical fibers fail from surface imperfections when placed under sufficient tensile stress. Accordingly, much effort has been devoted to the elimination of surface flaws by careful handling during and after lass forming by a protective plastic coating, and by various treatments to the glass surface. In the latter case, one method of reducing failure by surface flaws is to provide a compressive stress on the glass surface that counteracts applied tensile stresses.
It is well known that flaws in glass grow subcritically prior to failure when subjected to tensile stress in the presence of water, ammonia, or other corrosive agents. This phenomenon of subcritical crack growth in glass is known as fatigue and greatly impacts the long-term reliability of glass-based materials such as glass optical fibers. Therefore, the fatigue performance of optical fiber is especially important to the design of low cost fiber cables which have fewer strength members and less environmental protection than standard optical telecommunications cables.
It has been known for some time that the strength of a glass body may be increased by forming its surface region from a glass with a thermal coefficient of expansion that is lower than the thermal coefficient of expansion of the interior glass. As the combination is cooled from high temperatures, this configuration places the glass surface in compression, thereby inhibiting the formation and growth of cracks. See, for example, U.S. Pat. No. 3,673,049 to Griffen et al. and Krohn and Cooper, "Strengthening of Glass Fibers: I, Cladding," Journal of the American Ceramic Society, 52(12):661-664 (1969).
Numerous attempts have been made to create a strengthened optical fiber with such a compressive surface layer. See, for example, U.S. Pat. No. 3,884,550 to Maurer et al. and MacChesney et al., "Low Loss Silica Core-Borosilicate Clad Fiber Optical Waveguide," American Ceramic Society Bulletin, 52:713 (1973). U.S. Pat. No. 4,181,403 to Macedo refers to compression in a thin surface layer formed by "molecular stuffing" in fiber with a large optical core and very thin optical cladding. Some of these attempts involved the use of a TiO.sub.2 --SiO.sub.2 outer layer on the fiber, as its thermal coefficient of expansion is known to be less than that of SiO.sub.2. See, for example, U.S. Pat. No 4,184,860 to Schneider et al., U.S. Pat. No. 4,243,298 to Kao et al., and Japanese Patent No. 1,255,795 to Taka et al.
U.S. Pat. No. 4,184,860 to Schneider et al. ("Schneider") describes an outer TiO.sub.2 --SiO.sub.2 layer with 8 weight percent TiO.sub.2 surrounding a 15 weight percent TiO.sub.2 layer which is heat treated (by "tempering") to devitrify and partially separate and/or crystallize. This heat treatment of the 15 weight percent TiO.sub.2 intermediate layer is intended to raise the thermal coefficient of expansion so that it is substantially greater than the coefficient of the outer TiO.sub.2 --SiO.sub.2 layer, thereby putting the outer layer in compression. Thus, the Schneider fiber design relies on the 8 weight percent TiO.sub.2 outer layer to provide enhanced strength through compression.
Another study involved SiO.sub.2 --TiO.sub.2 glasses containing 10-20 weight percent TiO.sub.2 that were clear when formed but exhibited increased opacity from phase separation and anatase formation, along with large changes in thermal expansion, upon heat treatment at temperatures below the annealing point. See "Binary Titania-Silica Glasses Containing 10 to 20 weight percent TiO.sub.2," Journal of the American Ceramic Society, 58(5-6) (1976) and U.S. Pat. No. 3,690,855 to Schultz ("Schultz"). By studying the physical properties of these TiO.sub.2 --SiO.sub.2 compositions, Schultz described three glass forming regions as stable (0-10 weight percent), metastable (10-18 weight percent) and unstable (&gt;18 weight percent).
Some recent research has been directed toward understanding the mechanism of crack growth in SiO.sub.2 glass on the molecular level. See, for example, Michalske and Bunker, "The Fracturing of Glass," Scientific American, December 1987, pp. 122-129. The Michalske and Bunker paper presents an atomistic study of glass fracture in the presence of water, but is limited to homogeneous SiO.sub.2 glass. Additional research has been directed toward crack growth in continuous fiber filled composites. See, for example, Michalske and Hellmann, "Strength and Toughness of Continuous-Alumina Fiber-Reinforced Glass-Matrix Composites," Journal of the American Ceramic Society, 71(9):725-31 (1988).
Thus, it is known in the art that the addition of a TiO.sub.2 --SiO.sub.2 outer cladding layer to an optical waveguide fiber produces beneficial results. A primary focus of this prior work has been increasing the fatigue resistance of the resulting optical waveguide fiber. Much of this work has concentrated on the use of TiO.sub.2 --SiO.sub.2 outer cladding layers of relatively high thickness, with the minimum thickness of said layers being about 1 .mu.m. For example, U.S. Pat. No. 4.243,298 to Kao et at. discloses the use of 1-10 .mu.m thick TiO.sub.2 --SiO.sub.2 layers, with a preferred range of 1-5 .mu.m. U.S. Pat. No. 4,877,306 to Kar discloses TiO.sub.2 --SiO.sub.2 outer cladding layers about 2-3 .mu.m thick. Others have identified the range of 2-5 .mu.m. See, for example, U.S. Pat. No. 4,975,102 to Edahiro et al. and Oh et al., "increased Durability of Optical Fiber Through the Use of Compressive Cladding," Optics Letters 7(5):241-43 (1982).
U.S. Pat. No. 5,067,975 to Backer et al. ("Backer et al. '975") expressly discloses the use of TiO.sub.2 --SiO.sub.2 outer cladding layers in the range of about 1-3 .mu.m in thickness. Additionally, U.S. Pat. No. 5,140,665 to Backer et al. ("Backer et al. '665") claims an optical waveguide fiber with a TiO.sub.2 --SiO.sub.2 outer cladding layer, including an outermost cladding layer with thicknesses of less than 3 .mu.m, and further claims outermost cladding layer thicknesses of less than 1 .mu.m. U.S. Pat. No. 5,180,411 to Backer et al. ("Backer et al. '411"), a continuation-in-part of Backer et al. '975, discloses a method for manufacturing a fatigue resistant optical waveguide fiber with a TiO.sub.2 --SiO.sub.2 outer cladding layer, whereby a layer of TiO.sub.2 --SiO.sub.2 is deposited on, a doped SiO.sub.2 preform and the resulting preform is exposed to an atmosphere containing, chlorine and oxygen at a temperature of about 900.degree. to 1400.degree. C. After consolidation, the blank is drawn into an optical waveguide fiber with inhomogeneities in its outer TiO.sub.2 --SiO.sub.2 layer.
U.S. Pat. Nos. 5,241,615 and 5,318,613 to Amos et al. ("Amos et al. '615" and "Amos et al. '613") disclose an optical waveguide fiber with a very thin titania-silica outer cladding layer and a method for making it. The outer cladding layer of TiO.sub.2 --SiO.sub.2 glass has a thickness of less than 1 .mu.m and a TiO.sub.2 concentration that is less than or equal to about 10 weight percent.
The present invention is directed to an improvement upon the manufacture of optical waveguide fibers with a TiO.sub.2 --SiO.sub.2 glass outer cladding layer.